This invention relates to a thread trimmer mechanism provided under a sewing-machine table of a sewing machine for cutting thread leading to the cloth after a sewing operation, and especially to a thread trimmer mechanism that controls the length of the thread being cut.
Thread trimmer mechanisms are known in the art and, for example, one is disclosed in Japan Published Examined Patent Application No. 57-41958. In the mechanism, a looper is operated to form single-thread chainstitches. When the needle thread is cut, the looper holds the needle thread so that the needle thread will not be cut too short. The needle thread is thus prevented from coming out of the eye of the needle which would, of course, require re-threading of the needle.
A thread trimmer mechanism is also disclosed in Japan Published Examined Utility Model Application No. 63-37024. In this related-art reference, a thread supply device is provided for supplying a pre-determined amount of needle thread from a spool. In relation with the operation of the thread trimmer, the thread supply device supplies the predetermined amount of needle thread from the spool prior to the trimming operation.
Both these related-art references control the length of the needle thread when the needle thread is cut, thus preventing the needle thread from coming out of the eye of the needle. Neither of the references controls the length of bobbin thread supplied from a bobbin in a rotary hook assembly, however. Therefore, when the bobbin thread is cut in sewing machines employing these prior art techniques, the bobbin thread is excessively drawn from the freely-spinning bobbin as a result of the operation of the thread trimmer.
To prevent the bobbin in the rotary hook assembly from spinning, a bobbin presser lever is disclosed in Japan Published Examined Patent Application No. 52-426. When the bobbin thread is cut, the bobbin presser lever slides and contacts the bobbin, thus preventing the bobbin from rotating further. As a negative feature, however, when the bobbin presser lever presses the bobbin during the cutting of the bobbin thread, the needle thread caught by the hook of the rotary hook assembly engages with the bobbin presser lever before the needle thread forms a loop around a bobbin holder in the rotary hook assembly. Subsequently, the needle thread is pulled toward a thread trimming knife. Consequently, an excessive amount of the needle thread is supplied. The time required to supply the excessive amount of needle thread increases the time in a cycle of the automatic thread trimming operation. A large load is applied to the needle thread supply device. As a result, a long thread extends from the last stitch on the cloth, thereby impairing the appearance of the cloth and requiring an additional trimming operation.